1. Field of the Invention
The present invention relates to cables for transmitting electrical signals or power. The signals may be either analog or digital in nature. In particular, the present invention relates to extruded cables wherein the conductors consist of groups of round strands laid in parallel to form flat conductors with high flexibility and improved transmission characteristics, including reduced attenuation, noise and signal skew.
2. Description of the Related Art
The concept of increasing the mutual inductance of a cable to reduce its attenuation was originally disclosed in 1904 by Michael Pupin's U.S. Pat. No. 761,995 for the invention of the telephone loading coil. Essentially, the coils function is to increase the mutual inductance of the two conductors to reduce the inductive reactance of the circuit, thereby minimizing the frequency selective attenuation that had previously made long distance telephone communications unintelligible. The effectiveness of that invention, commonly referred to as the Pupin Coil, made it the worldwide standard for telephone systems over the past century.
In the many years since the development of the telephone loading coil, the bandwidth and dynamic range of audio systems have increased dramatically. To meet those demands, a number of cable designs have been developed to provide the benefits of both low inductance and low resistance without the complexity, cost and functional limitations of external loading coils. The challenge of those designs is to overcome the inherent tendency of thick cylindrical conductors to increase inductive reactance while creating additional frequency selective loss due to ‘skin effect’. To provide low loss from both inductance and resistance, a cable must have relatively thin conductors with a large cross-sectional area, and the distance between the two polarities must be minimized. While previous low-Inductance cable designs have provided some improvements over conventional cables, none have proven to be both highly effective and practical to implement in a wide range of applications. Therefore, the need exists for cables that can minimize both inductive and resistive losses, while maintaining the practicality of conventional cable designs.
The audio cable disclosed by Poulsen in U.S. Pat. No. 6,225,563 provides the signal transmission advantages of low inductance, but its applications are somewhat limited by its use of extremely thin ribbon conductors, which are fragile and require the use of special handling and termination procedures.
The low inductance loudspeaker cable disclosed by Goertz in U.S. Pat. No. 5,393,933, has one embodiment that utilizes solid flat conductors which are inherently stiff and require the use of specialized parts and techniques for termination and handling. Another embodiment disclosed in by Goertz utilizes stranded conductors, but it fails to provide any effective means for stabilizing the conductors when the cable is flexed.
The loudspeaker cable disclosed by Endo in U.S. Pat. No. 4,208,542 provides the benefits of low inductance through the use of a complex braided construction consisting of numerous enamel coated strands. The termination procedure for the Endo cable is very unusual and requires the use of specialized tools and instructions.
Nonetheless, there still remains a need for a cable that provides the signal transmission advantages found in the Paulsen, Goertz, and Endo cable designs, while also providing additional benefits such as lower noise and the versatility of conventional cable designs and extending those benefits to a wider range of applications.
Cables also degrade the fidelity of signal transmission by introducing noise. In addition to externally induced noise, or electromagnetic interference (“EMI”), cables contaminate electrical signals with triboelectric noise, which is generated by movement, intermittent contact and localized charge/discharge effects between the conductors and insulation. Several methods for minimizing triboelectric noise are disclosed in by Price in U.S. Pat. No. 3,433,687 and Lloyd in U.S. Pat. No. 4,486,252. The cable structure disclosed by Price utilizes semiconducting compounds and increased contact area between the conductor and insulation materials to minimize noise. The cable structure taught by Lloyd takes the Price concept a step further by imbedding the shield conductor in a conductive plastic compound. What is still needed, however, are cables that combine the advantages of reduced triboelectric noise with the improved transmission capabilities afforded by reducing inductance, while preserving the simplicity and low cost of conventional cable designs. The proliferation of high definition digital video and audio applications such as the HDMI (High Definition Multimedia Interface) is continuously increasing the demand for high-speed data cables with increased bandwidth and signal fidelity. In addition to the signal attenuation and triboelectric noise problems mentioned above, the performance of high-speed digital signal cables can be limited by several additional factors, including impedance uniformity, crosstalk, and skew. Therefore, the need exists for cables that can minimize those limitations to deliver data at higher speeds over longer lengths and with greater consistency.
The connection standards for the most common high-speed data applications utilize differential pairs, in which the data is represented by polarity reversals in the voltage applied to the cable by the sending device. The polarity reversals create signals in the form of high-frequency square waves. As the signal current is conducted through a cable, the signal waveforms are attenuated and distorted by the loss factors of the cable. The degree of waveform attenuation and distortion introduced by a digital signal cable has a direct influence on the number of data errors produced by the receiving device. Therefore, a cable design that minimizes all of the known loss factors would reduce data errors while allowing higher transmission speeds and longer cable lengths to be utilized. The primary advantage of utilizing differential connections is their superior ability to reject noise from external sources. Since the two sides of a differential signal are equal and opposite, noise picked up by the pair of conductors will tend to be eliminated by phase cancellation. Despite that distinct advantage over single-conductor cables, differential cables are nonetheless subject to a variety of limitations that can distort and contaminate both analog and digital signals.
Digital signal transmission is highly dependent on the accurate timing of the waveforms appearing at the receiving device. If one side of a differential signal arrives significantly ahead of the other side, the resulting waveform will be distorted. Timing errors in differential digital connections must be minimized to provide reliable and accurate signal transmission. The specific delay time between the signals received from the two conductors of a differential pair, or between two corresponding pairs, is called skew. The signal skew produced by a cable increases with the length of the cable. Skew can be caused by impedance variations or differences in the length of the conductors or conductor pairs. In high-speed data cables, minimizing signal skew is necessary to prevent the destructive digital timing errors known as jitter.
The measured loss of signal amplitude in a cable is called attenuation. Reducing cable attenuation is desirable, because it allows a cable to function properly over longer distances. The attenuation of a cable is primarily caused by resistance, inductance and capacitance, but variations in the loading effect of characteristic impedance along the length of a cable can also increase attenuation. Furthermore, a net difference in the impedance of the two conductors of a differential pair can also cause a skew error. Since those impedance variations are caused by inconsistent positioning of the conductors in relation to the shield, it is desirable for the conductors to maintain consistent positions with respect to one another and with respect to the cable's shield. It is also desirable for a cable to withstand the flexing and physical stress of long-term use and to maintain stable transmission characteristics as the cable is flexed.
The high-speed data transmission cable design disclosed by Kebabjian et al. in U.S. Pat. No. 6,403,887 provides a method of minimizing both impedance and length variations within a differential pair of conductors, in addition to stabilizing the conductor positions and impedance variations as the cable is flexed. While the design taught by Kebabjian makes progress in addressing these issues, cable structure designs that can further reduce noise, inductive loss and attenuation can provide higher performance in contemporary high-speed data applications.
The high-speed data transmission cable designs disclosed by Nair in U.S. Patent Application Publication Number 2008/0173464 utilize flat conductors each consisting of a single flattened wire in order to minimize loss due to ‘skin effect’. The single flattened wire design, however, not only adds specialized procedures to the manufacturing process, it creates a cable structure that is inherent more stiff, which would be a distinct disadvantage in speaker cable applications where flexibility and ease of termination are required. Furthermore, the shielded version of the Nair invention does not include any provision for a low loss ground path for the shields, which is a basic requirement for high speed, low loss data cables. The use of a conventional braided or served shield over each of the balanced pairs would negate most of the advantages of the design because of the inductive nature of those shields. Consequently, there remains a need for a cable design that can minimize loss due to skin effect, while still using standard round strands, so not to require additional manufacturing and so that the flexibility of the cable is preserved. Additionally, it would be desirable for the cable design to utilize drain wires and shield with conductive characteristics that match or even exceed those of the signal pair, which is common practice in conventional balanced pair shielded data cables.
Flattened conducting wires are also disclosed by Nair in U.S. Pat. No. 7,449,639, where the flattened conducting wires coated with insulation are bonded to one-another, with rectangular cross-sections and flat surfaces. Separate flat wire pairs are geometrically oriented within outer rectangular shell, and there is a separate core structure. In U.S. Pat. No. 7,462,782, Clark teaches of a number of different insulated conductors optimizing geometric shapes and forms for communication cables to enhance performance. There is no teaching in Nair or Clark, however, to use standard round strands that do not require additional manufacturing and that retain flexibility in the cable to create wires that are flat, rectangular or any other geometric shape.